The requirement for a stable reference voltage is almost universal in electronic design. Circuits that provide a stable reference voltage are used in dynamic random access memories (DRAMs), flash memories, analog devices, and other applications. These circuits require voltage generators that are stable over manufacturing process variations, supply voltage variations, and operating temperature variations and can be implemented without modifications of conventional manufacturing processes. Low voltage, battery operated circuits, operating at voltages as low as 0.9 volts (V), are becoming more common and also require stable, precise temperature-independent reference voltages.
The most common conventional reference circuit for low voltage applications is a bandgap reference circuit. The basic concept behind a bandgap reference circuit is to add a voltage with a positive temperature coefficient to a voltage with a negative temperature coefficient. When the two voltages are summed, the temperature coefficients cancel out each other, and the combined voltage source will be temperature independent.
Conventional silicon bandgap circuits suffer from an intrinsic limitation of having a minimum output voltage of approximately 1.25 V, i.e., the voltage of the bandgap of silicon. Today, this voltage acts as the lower limit on reference voltages for most applications.
There have been many attempts to overcome this 1.25 V limitation and create a sub-1.25 V reference circuit. However, conventional sub-1.25 V reference circuits suffer from a combination of high impedance outputs, increased current consumption, and a non-zero temperature coefficient in the sub-1.25 V region. Finally, most of these reference circuits are quite complex.
Accordingly, a need exists for a simple reference circuit, which can operate in the sub-1.25 V region while retaining substantial independence from temperature and process variations.
For simplicity and clarity of illustration, the drawing figures illustrate the general manner of construction, and descriptions and details of well-known features and techniques may be omitted to avoid unnecessarily obscuring the invention. Additionally, elements in the drawing figures are not necessarily drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help improve understanding of embodiments of the present invention. The same reference numerals in different figures denote the same elements.
The terms “first,” “second,” “third,” “fourth,” and the like in the description and in the claims, if any, are used for distinguishing between similar elements and not necessarily for describing a particular sequential or chronological order. It is to be understood that the terms so used are interchangeable under appropriate circumstances such that the embodiments of the invention described herein are, for example, capable of operation in sequences other than those illustrated or otherwise described herein. Furthermore, the terms “comprise,” “include,” “have,” and any variations thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements is not necessarily limited to those elements, but may include other elements not expressly listed or inherent to such process, method, article, or apparatus. The term “coupled,” as used herein, is defined as directly or indirectly connected in an electrical or non-electrical manner.